I want to detect the when the 'yes' radio button is chosen with JavaScript. This is a small lightweight form and does not have jQuery or other library installed. My goal is for it to work in most major browsers and be written in just a few lines of code.

I just want to note that your name makes this question priceless :p That said, jQuery is extremely lightweight and can be included without needing to host it yourself using google: code.google.com/apis/libraries/devguide.html#jquery. jQuery works on all major browsers.
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NaatanNov 10 '11 at 20:19